In Magnetic Resonance Imaging (MRI), the transmitter may be set to transmit at a particular RF frequency as a function of a specified polarization magnetic field strength. The frequencies to which the transmitter can be set are known as the Larmor frequencies, which are the NMR frequencies at which a nucleus precesses. The Larmor frequency is varied by a small amount .DELTA.F, to image NMR nuclei within a number of "slices" within an object (e.g., a human body).
In prior art systems, the production of the Larmor frequencies involved using analog local oscillator circuits which inherently have RF carrier leakage in the amplitude modulation circuit for the double side band (DSB) and single side band (SSB) modes, as well as unwanted side bands in quadrature amplitude modulation. Moreover, analog circuits are characteristically imprecise in the setting of parameters and have operating characteristics that are nonlinear and prone to drift with time and temperature.
A discussion of digital technology in MRI systems appears in an article by Holland and MacFall, "An Overview of Digital Spectrometers for MR Imaging" published in the March/April 1992 issue of JMRI, Volume 2, No. 2 at pages 241-246. Holland and MacFall conclude that the use of digital technology offers improvements in precision and accuracy that lead to improved fidelity of section profiles, dynamic range, image S/N, calibration and maintenance requirements. In FIG. 2 of the article a digital system of a simplified design is described. This system uses a digital synthesizer chip, consisting of input registers and a phase accumulator (NCMO), and a PROM to generate a digitized sine wave, and amplitude modulates digitally by multiplying the digitized sine wave by a modulation waveform.